Driving force control units are known for preventing deteriorated fuel consumption due to unnecessary driving force by way of decreasing driving force from a strong condition to a weak condition in response to the depression of a brake pedal when a transmission of a vehicle is selected to a driving range and the vehicle moves at extremely low speeds. For example, Japanese Patent Publication Nos.1-244930 and 9-202159 disclose a vehicle with such a driving force control unit. In these vehicles, driving force is decreased in any circumstances when the brake pedal is depressed.
However, a vehicle sometimes stops without depression of the brake pedal when backward displacement force derived from the vehicle's own weight is equivalent to driving force of the vehicle. When the vehicle advances forward on an up slope by the inertial force, the vehicle gradually slows down without depressing the brake pedal and at one point the vehicle starts to displace backwards. If driving force is exerted on the vehicle, the vehicle stands still for a while at the point with the driving force balanced with the backward displacement force. If the driver depresses the brake pedal so as to keep the vehicle stationary on the slope, a brake switch actuates in response to a slight depression of the brake pedal (before sufficiently depressing the brake pedal) and the driving force is instantly switched to the weak condition. Since the braking force corresponding to the slight depression of the brake pedal is insufficient, if the difference between the strong condition (strong creep condition) and the weak condition (weak creep condition) is set to be greater, backward displacement restriction force (total amount of the insufficient braking force and the driving force in the weak creep condition) is less than the backward displacement force derived from the vehicle's own weight. Therefore, the vehicle displaces backwards in a moment (for a very short while until the brake pedal is sufficiently depressed and braking force increases sufficiently) when the brake pedal is depressed.
The driver often drives a vehicle without operating the accelerator pedal so that the vehicle climbs a slope by the inertial force. When the vehicle reaches to a critical point at which the vehicle stops or slightly displaces backwards, the driver then depresses the brake pedal. If the driving force is instantly decreased by this brake pedal operation, the vehicle moves backwards against the driver's intention (irrespective of depression of the brake pedal). Such an unexpected backward displacement of the vehicle is not preferable.
The present invention seeks to provide a driving force control unit for a vehicle, which prevents backward displacement of the vehicle when depressing the brake pedal.